1. Field of the Invention
The invention relates to a method for manufacturing anchor bolts which are used to fix various mechanical parts, devices or the likes on a concrete structure or concrete body such as wall, ceiling, floor or the like, and also relates to an apparatus used in the method.
2. Description of Prior Art As shown in FIGS. 14 and 15, a prior art anchor bolt comprises an anchor body member 1 composed of a rod 2 provided with an expansion body 3 at its one end and a male screw thread 4 at its other end. A portion 5 of reduced diameter is formed adjacent to the expansion body 3, and an expansible sleeve 6 is mounted on the portion 5. The expansible sleeve 6 is made by die-cutting a thin metal plate and thereafter bending same into a cylindrical shape such that an outer diameter thereof is almost the same as that of the male screw thread 4. The expansible sleeve 6 is loosely fitted on the portion 5 of reduced diameter and has on its outer surface some scratching protrusions 7 which prevent the sleeve from being pulled out of a hole 9 in concrete body 8.
As is shown in FIG. 15, the anchor bolt is struck into the hole 9 of the concrete body 8, with its expansion body 3 positioned deepest in said hole. The scratching protrusions 7 of the expansible sleeve 6 engages with an inner surface of the hole 9. Then, a nut 10 put on the male screw thread 4 is tightly driven to secure an object 11 which is to be mounted on the concrete body. A pulling force thereby imparted to the anchor body 1 causes a tapered surface 3a of the expansion body 3 to forcibly expand the expansible sleeve 6, whereby said anchor body 1 is fastened to the concrete body 8.
The anchor bolt described above is simple in its structure wherein a diameter of the rod 2 of anchor body 1, a maximum diameter of the expansion body 3, the outer diameters of said male screw thread 4 and expansible sleeve 6 are almost the same. Therefore, the hole 9 drilled in the concrete body 8 can have a minimum diameter, which provides a great advantage to the related construction works.
It is, however, to be noted that the anchor body 1 has been manufactured from a raw rod 2A of a diameter equal to a diameter of thread of the male screw thread 4, by cutting or machining technique to form said enlarged head 3, portion 5 of reduced diameter and said male screw thread 4. Such a manufacture method is disadvantageous in that productivity is low, yield of good products is low and manufacture cost is high.
Recently, multistage cold-forging also has been applied to manufacture of the anchor body 1 in order to improve the productivity and to lower the manufacture cost. Manufacturing equipment for the multistage cold-forging is very expensive and needs many machine tools due to the multistage system so that a remarkable decrease in manufacturing cost cannot be looked for from this system.